This invention relates generally to fasteners and, more particularly, to a locking nut having a threaded insert.
Fasteners which are used to connect structural members together in critical applications often require a locking feature to prevent the fastener from becoming loose, or the structural members from becoming separated. Loosening of the fastener is not only a problem due to possible detachment of the structural members, but on vehicles such as spacecraft or aircraft, loose parts of the fastener and associated components may cause a hazard by interfering with the operation of equipment. In addition to requiring a locking feature, fasteners used in spacecraft have additional design restrictions. The fasteners should be free of external plating to prevent possible contamination from the plating flaking off the fastener or reacting with the environment in a space vehicle or habitat. Moreover, the fasteners should be free of any sharp protrusions which may interfere with equipment or possibly damage an astronaut's space suit.
Currently available locking fasteners typically include a nut N and bolt B for connecting two or more structural members M together (FIG. 1). The nut N includes a locking thread T which is formed by crimping the last several threads on the end of the nut to provide a tight interference fit between the threads on the shank of the bolt B and internal threads on the nut to prevent loosening of the nut on the bolt due to vibration. The remaining portion of the nut must have sufficiently thick walls to provide adequate strength for tightening the nut on the bolt. Therefore, only the end of the nut has a thin wall to allow for crimping of the threads to form the locking threads. The location of the locking thread at the end of the nut requires the threaded shank of the bolt to extend beyond the end of the nut to provide adequate engagement of the bolt threads with the locking threads located at the end of the nut. In space station applications this introduces a hazard due to possible snagging of the astronaut's space suit on the sharp threaded ends of the bolt extending from the nut. For this reason, the ends of the bolts are typically encapsulated with a plastic sealant after installation of the fastener. This is time consuming and increases the cost of the spacecraft since a large number of fasteners are used throughout the spacecraft.
Furthermore, the fasteners are typically plated to provide lubrication during installation and to prevent galling caused by moving engagement of threaded surfaces during installation. Plating is typically applied over the entire fastener, including the exterior to eliminate costs associated with applying plating only to the threaded portion of the fastener. The plating on the exterior of the fastener, however, increases the potential for contamination. The atomic oxygen in the orbital environment of a space station often causes plating on the outer surfaces of the fastener to become oxidized and creates a contaminant which may damage nearby equipment. Oxidized silver plating, for example, causes formation of a black dust which can spread throughout the space station.
A prior art design which can be used to eliminate the plating contamination problem is a thin walled insert, such as the type disclosed in U.S. Pat. No. 3,081,808, which is incorporated herein by reference. These inserts are used to provide a threaded bore for receiving a bolt in a large structural member. Since the structural member is not movable relative to the bolt and takes the place of the nut, the bolt must be accessible to torque the bolt into the threaded bore.